Each year, thousands of young adults sustain injuries to the joint surface, which can lead to premature arthritis, as articular cartilage does not heal. Thus, a method is needed for biological repair or regeneration of cartilage. The long-term objective of this work is to optimize cartilage repair by understanding how the cascade of events in neochondrogenesis (new cartilage formation) is regulated. For this to be achieved, a thorough understanding of the molecular mechanisms regulating cartilage formation and repair is required. Recent advances in tissue and the cell transplantation offer promise of hope for patients who could benefit from cartilage repair. The mechanisms by which undifferentiated mesenchymal cells (chondrocyte precursors) detect mechanical stimuli are unknown. Although chondrogenesis diminishes with age, it is clear that it is significantly enhanced through the use of mechanical stimuli, such as continuous passive motion. The investigators have developed a unique organ culture model using periosteal explants suspended in argarose in a mechanical environment. The unique feature of this model is that it mimics the in vivo repair of cartilage, from proliferation and differentiation of prechondrocytes, into chondrocytes and, thereafter, matrix synthesis. The goal of this grant proposal is to use this in vitro system to analyze the cellular responses of cartilage repair tissue to biomechanical stimuli using cyclical changes in hydrostatic pressure, which are referred to as dynamic fluid pressure (DFP). The project's Specific Aims are to (1) determine if cell proliferation is required for chondrogenesis; (2) characterize the phenotypic changes during DFP-mediated chondrogenesis; (3) identify mediators of DFP action during periosteal chondrogenesis; and (4) determine the age-related changes in the response of periosteum to DFP as a preliminary to understanding how and why the cartilage repair potential declines with age. It is suggested that the translational research proposed in this application is important because cartilage repair techniques, such as periosteal transplantation, are currently used to treat patients. Findings from these studies establish parameters for mechanical stimulation of tissue-engineered cartilage and a framework on which further research and tissue engineering of cartilage repair can be based, including how cells interact during paracrine regulation. It is felt that these findings will have relevance to other important area of orthopedics, including fracture healing, and have significant potential to impact the care of patients.